Ruben Bjørge

The Dual Nature of Precipitates in Al-Mg-Si Alloys

Precipitates in Al-Mg-Si-(Cu) alloys all contain a similar hexagonal arrangement of Si-atoms. Precipitates come and go but their inner Si ordering appears to vary little throughout the precipitation process. In order to improve understanding of precipitation and the related material properties, it is becoming increasingly clear that this includes a good understanding of the hexagonal Si-network, its relation to the precipitates and the Al matrix. Previous studies have revealed that adding Cu atoms to the ternary system, causes the Si network to twist slightly in the matrix about its hexagonal axis, favoring different precipitates. Here we investigate these two rotations. It is shown they can be viewed as a mirror of the network itself about a {310} Al plane. Since precipitates are coherent, the Si-network with its triangular arrangements of Si must also match a fourfold arrangement of Al on the {100} planes. Sets of Al lattice positions exist which can approximate the tree-fold Si symmetry, according to the experimentally observed orientations, and one or more large super-cells can be found having near fit in both lattices. The mirror plane is a main plane in one such super-cell. We show that the mirror leaves every seventh node of the network unchanged, thus defining a smaller hexagonal super-cell in the network, similar to the B’ or Q’/Q phase, where corners are invariant, but where the Si contents is flipped.

Microstructure, hardness evolution and thermal stability of binary Al-7Mg alloy processed by ECAP with intermediate annealing

Abstract A binary Al-7Mg alloy was processed by equal channel angular pressing (ECAP) at room temperature via route Bc, combined with intermediate annealing. After 6 passes, a high hardness of 218 HV is achieved. Transmission electron microscopy (TEM) observations demonstrate that ECAP leads to a significant grain refinement and ultrafine grains down to 100-200 nm are developed after 5 or 6 passes. X-ray diffraction (XRD) analysis indicates that the major part of Mg atoms are in solid solution in the deformed material, and the possible strengthening effect of Mg solute atom clusters or precipitates is neglected. The high hardness of the 6 pass-treated materials comes mainly from grain boundary strengthening, which contributes about 41% to the total strength, while dislocations and Mg solid solution contribute about 24% and 35% to the remaining strength, respectively. Also, the thermal stability of this severely deformed material was investigated by hardness measurements. The material is relatively stable when annealed at a temperature lower than 250 °C, while annealing at 300 °C leads to a rapid softening of the material.

Formation of incoherent deformation twin boundaries in a coarse-grained Al-7Mg alloy

Deformation twinning has rarely been observed in coarse grained Al and its alloys except under some extreme conditions such as ultrahigh deformation strain or strain rates. Here, we report that a significant amount of Σ3 deformation twins could be generated in a coarse-grained Al-7 Mg alloy by dynamic plastic deformation (DPD). A systematic investigation of the Σ3 boundaries shows that they are Σ3{112} type incoherent twin boundaries (ITBs). These ITBs have formed by gradual evolution from copious low-angle deformation bands through 〈111〉-twist Σ boundaries by lattice rotation. These findings provide an approach to generate deformation twin boundaries in high stacking fault energy metallic alloys. It is suggested that high solution content of Mg in the alloy and the special deformation mode of DPD played an important role in formation of the Σ and ITBs.

In-situ X-ray tomography study of cement exposed to CO2 saturated brine

For successful CO2 storage in underground reservoirs, the potential problem of CO2 leakage needs to be addressed. A profoundly improved understanding of the behavior of fractured cement under realistic subsurface conditions including elevated temperature, high pressure and the presence of CO2 saturated brine is required. Here, we report in situ X-ray micro computed tomography (μ-CT) studies visualizing the microstructural changes upon exposure of cured Portland cement with an artificially engineered leakage path (cavity) to CO2 saturated brine at high pressure. Carbonation of the bulk cement, self-healing of the leakage path in the cement specimen, and leaching of CaCO3 were thus directly observed. The precipitation of CaCO3, which is of key importance as a possible healing mechanism of fractured cement, was found to be enhanced in confined regions having limited access to CO2. For the first time, the growth kinetics of CaCO3 under more realistic well conditions have thus been estimated quantitatively. Combining the μ-CT observations with scanning electron microscopy resulted in a detailed understanding of the processes involved in the carbonation of cement.

Germanium network connecting precipitates in an Mg-rich Al-Mg-Ge alloy.

Precipitates in an Al-0.87Mg-0.43Ge (at.%) alloy, heat-treated for 16 h at 200 degrees C, were investigated by transmission electron microscopy and annular dark-field scanning transmission electron microscopy (ADF-STEM). Earlier studies of Al-Mg-Si-(Cu) have shown that an Si network exists within all precipitates. Here, it was investigated whether the heavier, more easily detectable germanium atom would behave similarly. The precipitates were more similar to those found in Al-Mg-Si-Cu alloys with a high fraction of disordered phases than to ternary Al-Mg-Si. All precipitate cross-sections along [001]Al imaged by ADF-STEM showed that Ge atoms arrange in triangular columns separated by approximately 0.4 nm. Along these columns, the precipitates 0.405-nm periodicity and coherency (along its needle axis) imply a Ge plane periodicity of 0.405 nm. A germanium network, therefore, exists in all precipitates in this alloy, with a hexagonal sub-cell (SC) a = b approximately 0.4 nm, c = 0.405 nm, which is very similar to the Si network in Al-Mg-Si-(Cu). The network always appears as ordered. Disorder in a precipitate must, therefore, be caused by the other atoms in the structure between Ge atoms. One difference between precipitates of the ternary systems Al-Mg-Ge and Al-Mg-Si is the orientation of the diamond element network (SC) base in {001}Al. In Al-Mg-Ge, a <100>SC edge falls along <100>Al. This coincides with the orientation in some precipitates in quaternary Al-Mg-Si-Cu. In ternary Al-Mg-Si, one SC base is parallel with a <510>Al direction.

Aberration-Corrected STEM Study of Precipitates in an Al-Mg-Si-Ge-Cu Alloy

Precipitation in a Mg-rich Al-Mg-Si-Ge-Cu alloy was investigated using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. The precipitates were needle or lath shaped with the longest dimension parallel to Al. The precipitates had no repeating unit cell when viewed along this direction. However, the precipitate structure in projection consisted of a hexagonal network of mixed Si and Ge columns, with Mg, Al, and Cu columns occupying specific sites in between the network columns. The Cu columns appeared with the same local arrangement of atomic columns as in Al-Mg-Si-Cu precipitates, and the Cu-free regions consisted of structural units with Mg and Al at specific sites. The proposed atomic model is supported by image simulations.

Precipitation in an Al-Mg-Ge Alloy

The precipitation in an Al-0.87 at% Mg-0.43 at% Ge alloy has been investigated using high-resolution transmission electron microscopy (HRTEM) and annular dark field scanning TEM (ADF-STEM). This work is a continuation of previous studies on AlMg-Si-(Cu) alloys, which are industrially relevant due to their superior mechanical properties such as high strength/weight ratio. The hardness increase is generated by the precipitation of nanometre-sized metastable phases from solid solution during artificial ageing. Most precipitates have had their crystal structures solved by our group using a combination of quantitative electron diffraction, first-principles methods and HRTEM [1-3]. It has been shown that all phases are structurally related through a near-hexagonal Si network having sub-cell dimensions a=b=0.405 nm, c=0.405 nm [3]. A possible way of improving the mechanical properties of Al-Mg-Si-(Cu) alloys is by replacing one or more solute elements with other ones having similar electrochemical properties. Total or partial replacement of Si by Ge is one such example. In bulk form Ge is similar in size to, and forms the same diamond structure as Si. An improvement in peak hardness and age-hardenability by replacing Si with Ge has previously been reported for an Al-1.0 wt% Mg2Ge alloy [4]. The higher atomic number of Ge with respect to Al and Mg makes Al-Mg-Ge alloys well suited for ADF-STEM studies. In the present work, hardness measurements indicate a moderate increase in strength and better stability during ageing for the Al-Mg-Ge alloy compared to a similar alloy containing Si instead of Ge. HRTEM and ADF-STEM images from early precipitates in the Al-Mg-Ge alloy show that Ge columns arrange in a near-hexagonal network with similar dimensions as the previously reported Si-network in Al-Mg-Si-(Cu) alloys. The Ge-network has directions parallel to Al (Figure 1), producing precipitates that sometimes have plate-like morphologies. In most precipitates the atomic arrangement on the Ge-network was disordered, but a particle with a monoclinic unit cell was also found (Figure 2). This type of atomic arrangement has only been observed in the AlMg-Si-Cu alloys [5].